Plural feed-back system for light sources



Nov. 13, 1945. w; R. LE PAGE 2,388,854

PLURAL FEEDBACK SYSTEM FOR LIGHT SOURCES I 2 Sheets-Sheet 1 Filed April 30, 1945 Wizeuefi. 559165,

Way/W ATTORNPEY.

Nov. 13, 1945. W. R; LE PAGE 1,388,854

PLURAL FEEDBACK SYSTEM FOR LIGHT SOURCES Filed April 30, 1945 2 Sheets-Sheet 2 A-c., p-c 6 4 1 AMPL/F'IEIQY 4 INVENTOR.

ATTORN EY.

Patented Nov. 13, 1945 PLURAL FEED-BACK SYSTEM FOR LIGHT SOURCES Wilbur R. Le Page, Pikesville, Md., assignor to V Radio Corporation of America, a corporation of "Delaware Application April 30, 1943', Serial No. 485,256 15 "Claims. (Cl. 250-415) This invention relates to electrical, current transmission and feedback systems, and particularly to electrical feedback circuits in combination with optical feedback circuits. The invention is particularly useful for controlling photographic sound recording and reproducing systems.

It is well known that in photographic sound recording systems, light sources or lamps are employed to produce light beams which may be either directly modulated by varying the energy supplied to the lamps in accordance with the variations of-sound waves to be recorded, or which may be modulated by vibrating the beam in accordance with the sound waves. Light sources of constant intensity are used for recording both variable area and variable density sound tracks, the light being modulated by the vibrations of a galvanometer mirror. Light sources-such as glow lamps, which vary in intensity are also employed for recording variable density records. For reproducing either type of record, a light source of constant intensity is used to produce the scanning beam.

In the absence of a control voltage such as noise reduction voltage, the average light output of the source should remain constant and uniform during the recording or reproducing period to prevent undesired variations in the sound levels or modulations and the introduction of distortion in the recorded or reproduced sound. With a noise reduction voltage, the light output should remain constant momentarily at a value determined by this voltage. Unwanted light variations may be produced by fluctuations in the lamp energizing source, by deterioration of the light producing elements. by the blackening of the lamp envelope, by the non-linearity of the light output with impressed voltage, and other reasons. It has been found that the maintenance of a constant voltage at the lamp terminals does not compensate for the above-mentioned causes of light variations. 4

The present invention, therefore, is directed to a system for stabilizing, controlling, and modulating the average light output by detecting a portion thereof and regulating the energy supplied to the lamp in accordance with the variations in the detected light. The general principle of such an optical feedback stabilizing system is known in the art. In certain of these prior systems which provide both direct current and alternating current feedback, the impedance of an electronic device connected in series with the lamp and the primary power source is varied in accordance with the light fluctuations.

the feedback arrangement making it possible to neutralize the primary and secondary ampere turns to avoid saturation of the transformer magnetic path.

The principal object of the invention, therefore, is to improve electrical current feedback systems.

Another object of the invention is to provide a combination electrical and optical feedback system in which linearity is provided in all portions thereof.

A further object of the invention is to provide in a sound recording or reproducing system a plurality of feedback paths, some of which are optical to provide linearity between light output and impressed voltage and others of which are electrical to provide linearity in theelectrical transmission elements.

A furtherjoblect of the invention is to provide an electrical feedback system in which the direct current component in one winding of a couplin transformer is controlled by the current in the other winding of the transformer to maintain a substantially constant average flux density.

A further object of the invention is to provide in a photographic sound recording system an optical'altemating current feedback path for rapid variations in light, an optical direct current feedback path for slow variations in light, an electrical feedback path for obtaining linearity in a vacuum tube transmission element in the alterhating current feedback path, and an electrical feedback path for obtaining linearity in a transformer for the alternating current feedback path.

Although the novel features which are believed to be characteristic of this invention are pointed out with particularity in the claims appended hereto, the manner of its organization and the mode of its operation will be better understood by referring to the following description, read in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic circuit diagram of one embodiment of the invention, and

Fig. 2 is a schematic circuit diagram of a second embodiment of the invention.

Referring now to Fig. 1, the invention is shown embodied in a sound recording system wherein a lamp 5 may be a glow tube which is adapted to be modulated by sound waves to be recorded when impressed across terminals 8. A portion of the light from source 5 is impressed on a film 5 1 .0 2 ShQWh) in any suitable well-known manner,

2 while a portion thereof is detected by a photoelectric cell 8 polarized from a suitable source of energy 'such as a battery 9.

The sound waves to be recorded impressed across terminals 6 are amplified by an amplifier II, the output of which is coupled to a second denser 22.

The impedance of the gas rectifier i8 is controlled by a direct current amplifier 23 having a plate supply 24, a coupling resistor 28 and a coupling bias battery 2'. The input of the tube 28 includes a resistor 29, a resistor 30, and a condenser 2I, across which the slowly varying feedback control voltage from the lamp appears, resistor fl and condenser 3| forming a filter for the modulation components oi the output voltage of the photoelectric cell 8 appearing across resistance 33. Resistor 29 is included to indicate one means of introducing a control voltage for the average light. such as a ground noise reduction controLthe voltage being applied across this resistor. A bias battery for the tube 23 is provided at It in the connecting conductors 35. Thus, for slowly varying lamp outputs, the direct current feedback circuit from the cell 8, through the amplifier 23 to gas tube II, will maintain uniformity thereof, since, for increases of light, a corresponding decrease in power supplied to the lamp results. For instance, should the average light output from the cell 5 increase, the bias on tube 23 will be made correspondingly more positive over conductors 2i andthus increase the bias on gas rectifier l8 by increasing the voltage drop across resistor 25. The increase in the impedance of tube It decreases the energy from source IT to the lamp 5 to bring its output back to the pre determined value. For decreases in the average light Output of the lamp 5, the reverse control is efiective for increasing the energy supplied to the lamp.

A high frequency alternating current feedback path is also provided from the photoelectric cell 8 over conductors 3B, conductors 36, condenser the complete feedback ath including the ampliilers II and I2. It is well known that such a feedback path will provide linearity between the light variations and the applied voltage produced by the sound waves as disclosed in Balsley Patent No. 2,242,638 of May 20, 1941 and copending application of H. I. Reiskind, Ser. No. 470.663. filed December 30, 1942. the latter specifically disclosing an alternating current feedback path and a separate direct current feedback path. In the application of noise reduction in a glow lamp system, there will be considerable variations in the 31, and resistance 38, to the input of amplifier ii,

direct current component of the lamp current and consequently in the secondary winding of the transformer l5. This will cause saturation difficulties unless the transformer is excessively large. However, these difficulties can be overcome by the use of two electrical feedback. circuits, now to be described.

It is m be noted that the plate circuit of am- As the lamp current is increased the current through the secondary winding of transformer I! is increased and the voltage drop across resistor 20 is increased, thus making the grid of amplifier l2 more positive. This action increases the plate current of amplifier l2 linearly with grid voltage because of the feedback above described. Now, the transformer is so connected that these increased plate currents flow around the transformer core in opposite directions with respect to the magnetic circuit to those in the secondary, thus maintaining a substantially uniform flux density in the core to prevent saturation. Thus, amplifier l2 should be large enough to carry a direct current greater than the peak of the modulations in order to provide for this fluctuation of the direct current component. With resistor ll large enough to make amplifier 12 linear down to cut-off, then the gain will be substantiall unity, if the voltage across resistor; ll is'considered the output and the voltage across resistor 2|! is considered the input. In this case the proper relation between resistors 4| and 20 is Ran/Rio =Ni/N2, where N1 and N: are the turns of the primary .and secondary windings, respectively, of transformer l5, and the circuit is adjusted by the bias on the grid of tube H, by the battery 42 or its equivalent, so that when the current in resistor 20 is zero, the plate current of amplifier i2 is zero.

It may be impossible or inconvenient, due to other circuit considerations, to have this amount of feedback, although the feedback should be enough to provide linear operation of amplifier i2 over the necessary range which, in terms of plate current of tube l2, will be Na/Ni times the range of slow variations of average lamp current. If A is considered the direct current gain of tube I! (gain being the ratio of direct current output increments to direct current input increments when the increments are taken at a slow rate), over the range of linearity and considering the voltage across resistor ll as the output voltage, then the relation between resistors 41 and 20 will be R4l/R20=A(Nl/N2) and the bias of tube i2 will be adjusted to make the product of the average lamp current and N2 equal to IlZNl anywhere within the range of linearity, I12 being the average plate-current of tube l2. This requirement of linearity extends only over the range of excursion of the average lamp current andnot the larger excursions of the modulation, this latter being controlled by the alternating current feedback path.

To illustrate this, assume that a modulation were suddenly applied and for simplicity that the noise reduction voltage remained constant. The alternating current feedback path will cause the light to be without distortion. but if there is nonlinearity in the lamp, the lamp current may have a different average value than before. The direct current feedback through conductors ll and tubes 23 and II will insure the maintenanc of the average light at a constant value and will. therefore, act to give this new correct average tional change in the average light at the same.

time the required change take place in the plate current of tube l2. The performance of the alternating current feedback continues as before, the only change being in the average plate current of tube l2 and in the average lamp current.

Thus, the above ystem provides a direct current feedback path to take care of slow variations in light output to maintain the average light output substantially constant, except when controlled by a signal such as a noise reduction voltage, a separate alternating feedback path for rapid or high frequency variations of light output to maintain a linear relationship between light output and impressed voltage, and two electrical feedback paths to provide proper magnetic cancellation of the D.-C. components in the transformer windings, to be applied in a transformer coupled light source. The D.-C'. feedback also provides linearity of average light with noise reduction, or any other control voltage when the circuit is used for other purposes than sound recording, This plural feedback system thereby overcomes the deleterious effects of all causes of unwanted light variations which introduce distortion in the recorded sound, provides a particularly stable glow lamp outputand maintains linearity in the electrical transmission circuit. It will be realized that the electrical feedback paths are applicable to other electrical current transmission systems without optical feedback, such as a system requiring the modulation and control of the direct current component of a large current through any element which might be substituted for the glow lamp 5.

Referring now to Fig. 2, in which the same elements of Fig. 1 are identified by the same numerals, a somewhat simplified system is shown wherein the alternating current and direct current feedback paths from the photoelectric cell 9 have been'combined'both paths now including a main amplifier 45 which must be a direct current as well as an alternating current amplifier, as shown. This combination feedback path includes the photocell 8, battery 9. and load resistor 99 of the system of Fig. l, the feedback currents being transmitted over conductors 46 to the input of amplifier 46. The energizing circuit for the lamp again utilizes the power-frequency alternating current source I! which is connected through the gas tube rectifier l8, resistor 20, and through, the secondary winding of transformer IS. The primary of the transformer I5 is in the output circuit of the main amplifier 45. The same direct current amplifier 23, with its plate potential supply 24, coupling resistor 25, coupling asses bias battery 26, and grid bias battery 94, is shown for controlling the rectifier l8. Resistor 29 provides a feedback voltage which is impressed on the grid of tube 23. The action of this circuit will now be described.

As the direct current component detected by cell 9 and amplified by amplifier '45 now appears across a resistor 49, it will be impressed over current of the transformer conductors ill on the input of the direct currentv amplifier 23 and will function negatively to maintain the average lamp output substantially constant in the absence of a control signal such as a noise reduction voltage in the same manner as when the output circuit of cell 8 was connected directly to the input of amplifier 23, as in Fig. 1. The saturation control circuit for the transformer 19 produced by the voltage appearing across resistor 49 will now be applied to amplifier 23, making the direct current component of the lamp current linear with th voltage across resistor 49, in view of the feedback provided by resistor 29. As in Fig. 1, the direct current components will flow in opposite directions, magnetically.

To further explain this circuit modification, if there is sufilcient feedback to make tubes 23 and I8 (considered as a direct current amplifier with the voltages across resistors 49 and 20 considered as the input and output, respectively) linear down to zero current, then the gain will be substantially unity and R49/R2o=N1/N2 is the correct relation between the values of resistors 49 and 20. In this case battery 26 will be adjusted to make the lamp. current zero-when the .primary I5 is zero. This amount of feedback may not be practical or necessary so let A be the overall D.-C. incremental gain of tubes 23 and I9 in the range of linearity as described above. Then resistors 49 and 20 will satisfy the relation R49/R2o=(1/A)(N1/Nz) and battery 28 will be adjusted to make the ampere turns of the direct current components of the primary and secondary of transformer l 5 balance one another at any point in the range of linearity of the combination of tubes 29-and l8.

To illustrate the action, let us assume-a modulation is suddenly applied with no change in noise reduction voltage. The alternating current modulation is transmitted by conductors 46, the amplifier 45, and the transformer I5. If there is nonlinearity between the light output and lamp current, the average current to keep constant average light, may change, and this current change is provided by the. same feedback path including tubes 23 and I8 'and associate circuits instead of transformer: Hi. When the noise reduction voltage is superimposed on the signal, the current is transmitted as in a conventional direct current feedback amplifier. The feedback will always maintain the averagelight at the new value which is determined by the new value of the noise reduction voltage, and will always maintain the correct ratio of D.-C. components of primary and secmitted by the transformer l5, while currents corresponding to slow variations in light are transmitted by the potential drop across resistor 49 impressed on direct current amplifier 23 and rectifier l8. .Thus, the control is from primary to secondary for both components. A modification of the system of Fig. 2 would be to use separate direct current and alternating current amplifiers for the main amplifier 45 or only a part of the main amplifier used by both direct and alternating currents.

Although the same general current neutralizing feature is found in both the systems of Figs. 1 and 2, it-is to be noted that in Fig. 1 the direct current component in the secondary winding of transformer l5 controls the direct current component in the primary winding thereof, while in Fig. 2 the direct component of primary current cuit of the direct current amplifier 23, and a similar resistor 52 and condenser 58 are provided in the input circuit of the amplifier 21 in Fig. 2, these elements preventing the modulating currents from being impressed on the gas rectifier tube i8, which is slow acting. Even if this-tube were fast-acting, this filter is desirable to prevent variations in the lamp current circuit from affecting the modulating current. Furthermore, batteries have been shown at each point where 9. voltage or current source is needed for simplifying the disclosure, but it is to be understood that bias resistances in the plate circuits of the tubes or any well known system for energizing from a common power source may be used. Also that a direct current amplifier of two or more tubes may be used in place of the single tube 2: in both circuit modifications, it being necessary, of course, to maintain the same phase relationship between input and output in the multiple stage amplifier as in the single stage.

I claim as my invention:

1. In a light source stabilizing controlling and modulating system, the combination of a source of light, an electrical energy supply for said source, means for detecting a portion of the light output from said source, a direct current amplifier, an alternating current amplifier, means for simultaneously impressing the output of said photoelectric pickup device on said amplifiers. said direct current amplifier transmitting currents corresponding to the slow variations in the light output to said electrical energy supply and said alternating current amplifier transmitting currents corresponding to the rapid variations in the light output of said light source, a transformer for connecting said alternating current amplifier to said light source, and an electrical coupling circuit between the primary and secondary windings of said transformer for maintaining proportionality between the direct current components in said windings and thereby obtaining linearity of transmission of said alternating currents through said transformer,

2. In a light source stabilizing, controlling. and modulating system, the combination of a source of light, anelectrical energy supply for said source, means for detecting a portion of the light output from said source, a direct current amplifier, an alternating current amplifier, means for simultaneously impressing the output of said photoelectric pickup device on said amplifiers, said direct current amplifier transmitting currents corresponding to the slow variations in the light output to said electrical energy supply and saidalternating current amplifier transmitting currents corresponding to the rapid variations in the light output of said light source, a transformer for connecting said alternating current amplifier to said light source, andan electrical coupling circuit for maintaining linearity of transmission of said alternating currents through said transformer, said electrical coupling circuit including a feedback circuit between the output and input circuits of said alternating current amplifier for maintaining linearity in said alternating current amplifier during current variations caused by said electrical coupling circuit.

3. In a light source stabilizing, controlling, and modulating system, the combination of a source of light, an electrical energy supply for said source, means for detecting a portion of the light output from said source,-a direct current amplifier, an alternating current amplifier, means for simultaneously impressing the output of said photoelectric pickup device on said amplifiers, said direct current amplifier transmitting currents corresponding to the slow variations in the light output to said eletcrical ener y supply and said alternating current amplifier transmitting currents 4. In a light sourcestabilizing, controlling, and modulating system, the combination of a source of light, an electrical energy supply for said source, means for detecting a portion of the light output from said source, a direct current amplifier, an alternating current amplifier, means for simultaneously impressing the output of said photoelectric pickup device on said amplifiers, said direct current amplifier transmitting currents corresponding to the slow variations in the light output to said electrical energy supply and said a1- ternating current amplifier transmitting currents corresponding to the rapid variations in the light output of said light source, a transformer forconnesting said alternating current amplifier to said light source, and an electrical coupling circuit for maintaining linearity of transmission of said alternating currents through said transformer, said electrical coupling circuit increasing the direct current components through the secondary of said transformer, as the primary direct. current components increase, in proportion to the turns ratio between said primary and secondary.

5. A feedback system comprising an electrical load, an electrical energy source, a transformer, means for connecting said load, electrical energy source and the secondary winding of said trans- I transformer in proportion to the turns ratio be- 5 the primary winding of said transformer, and

feedback means for simultaneously increasing and decreasing the direct current components of the currents in said primary and secondary windings in opposite magnetic directions with respect to the core of said transformer in proportion to the turns ratio between said windings, the direct current component in one winding controlling the value of the direct current component in the other winding. I

7. A feedback system comprising an electrical load, an electrical energy source, a transformer, means for connecting said load, electrical energy source and the secondary winding of said transformer in series, a, low frequency feedback circuit for controlling the average value of the current in said series circuit, an amplifier connected to the primary winding of said transformer, and feedback means for simultaneously increasing and decreasing the direct current components of the currents in said primary and secondary windings in opposite magnetic directions with respect to the core of said transformer in proportion to the turns ratio between said windings, a coupling circuit being connected between said series circuit and the input of said amplifier, variations in the direct current component in said secondary winding controlling the current in said primary windin 8. A feedback system comprising an electrical load, an electrical energy source, a transformer,

means for connecting said load, electrical energy source, and the secondary winding of said transformer in series, a low frequency feedback circuit for controlling the average value of the current in said series circuit, an amplifier. connected to the primary winding of said transformer, and feedback means for simultaneously increasing and decreasing the direct current components of the currents in said primary and secondary windings in opposite magnetic directions with respect to the core of said transformer in proportion to the turns ratio between said windings, a coupling circuit being connected between the output circuit of said amplifier and said means for controlling the average value of the currents in said series circuit for varying the direct current component in said secondary winding in accordance with the variations in the direct current component in said primary winding.

9. A feedback system comprising an electrical load,an electrical energy source, a transformer, means for connecting said load, electrical energy source, and the secondary winding of said transformer in series, a low frequency feedback circuit for controlling the average value of the current in said series circuit, an amplifier connected to the primary winding of'said transformer, and feedback means for simultaneously increasing and decreasing the direct current components of the currents in said primary and secondary windings in opposite magnetic directions with respect to the core of said transformer in proportion to the turns ratio between said windings, a feedback circuit being connected between the output and input circuits of said amplifier for obtaining linearity in said amplifier.

10. In combination, alight source, an electrical energy supply for said source, a control means for controlling the electrical energy supplied to said source from said electrical supply, a photo-sensitive device for detecting a portion of the light from said source, means for interconnecting said device and said control means for maintaining the average value of the light output substantially means for maintaining linearity between the light output of said source and aninput voltage to said interconnecting means, and means for interconnecting said device and said light source for maintaining linearity between the light output therefrom and an input voltage to said first mentioned interconnecting means, and means including a resistor common to the circuit of said control means and said second mentioned interconnecting means, for maintaining linearity within said last mentioned interconnecting means.

11. In combination, a, light source, an electrical energy supply for said source,'a control means for controlling the electrical energy supplied to said source from said electrical supply, a photo-sensitive device for detecting a portion of the light from said source, means for interconnecting said device and said control means for maintaining the average value of the light output substantially constant, said interconnecting means including means for maintaining linearity between the light output of said source and an input voltage to said interconnecting means, and means for interconnecting said device and said light source for maintaining linearity between the light output therefrom and an input voltage to said last mentioned interconnecting means, and means for constant, said interconnecting means including maintaining linearity within said last mentioned interconnecting means, said first mentioned interconnecting means including a direct current amplifier connected between the output circuit of said device and said control means, and said second mentioned interconnecting means including an alternating current amplifier connected between the output circuit of said device and to said source, the variations in the direct current component through said source controlling the direct current component in the plate circuit of said alternating current amplifier.

12. In combination, a light source, an electrical energy supply therefor, a control device for said electrical energy supply, a detector for a portion of the light from said source, an amplifier connected intermediate said detector and light source for currents corresponding to the variations in the light output of said source, interconnecting means for impressing the rapid variations in said currents directly on said source and for impressing the slow variations in said currents on said control device and means for maintaining linear-' ity of transmission of said rapid current variations, said last mentioned means connecting the output of said amplifier with the input of said control device to vary the direct current component through said source in proportion to the variations in the direct current component in the output circuit of said amplifier.

13. In a plural channel feedback system for stabilizing the output of a light source, the combination of a. detector for a portion of the light from said source, an electrical source of energy for said light source, an energy supply circuit between said light source and said energy source, a feedback path from said detector to said supply circuit for varying the impedance of the supply circuit, a second feedback path from said detector to said light source for varying the alternating voltage impressed on said light source, and means including a common portion of said supply circuit and said second feedback path for maintaining linearity of said second feedback path during variations in the average current transmitted thereby.

14. A plural channel feedback system in accord ance with claim 13 in which said second feedback path includes an alternating current amplifier, a transformer. a feedback circuit between the output and input circuits of said amplifier, and an interconnecting circuit including a, common portion of said supply circuit and the'output circuit of said amplifier between the seconda y and primary windings of said transformer through said amplifier.

15. In a plural channel feedback system for stabilizing the output of a light source, the combination of a detector for a portion of the light from said source, an electrical source of energy for said light source, an energy supply circuit between said asaaass 

